In an e-mail system, a mail receiver sorts incoming mails according to the transmission time and date and the reception time and date, and displays them to a recipient. A recipient generally opens mails in a display order and refers them. Because of this, there is a possibility for a mail to be left over without opening by a receiver depending on where the mail is located in a display order, or to be put off for the circumstances of a receiver when the number of received mails is large. In these cases, there is a possibility that the information, which a sender aims to notify by a certain time limit, cannot be notified by this time limit. On this account, there is a fear which the effectiveness of the information, which a mail has, is lost. Even if a mail is opened, a recipient cannot find a sender's expectation about time limits for opening and referencing of the mail until a recipient open the whole part and read the whole length. On this account, the recipient needs to open the whole part of a mail and refers to it for getting the intention of a sender on the time limit for opening of this mail, and this process will cause a heavy burdens to a recipient.
E-mail systems, which handle this problem, are disclosed by Japanese Patent Application Laid-Open H7-162452 and Japanese Patent Application Laid-Open H7-183912. An e-mail system disclosed by Japanese Patent Application Laid-Open H7-162452 includes a mail transmitter which adds a time limit for opening to a mail and transmits, and a mail receiver which receives a mail from the mail transmitter. A mail receiver refers to a time limit for opening which was added to the received mail from the mail transmitter, and sorts the received mails, which are left non-opened, in the order that the one which a time limit for opening has passed is placed prior to the one which a time limit for opening is approaching, and displays them in the order. Also, the mail receiver displays alarms for distinguishing the mails which a time limit for opening has passed and the mails which a time limit for opening is approaching. By this, a recipient can open and refer a received mail while taking into account the priority.
An e-mail system disclosed by Japanese Patent Application Laid-Open H7-183912 provides a mail transmitter which adds the reply necessity of a recipient with a time limit for opening to a mail and transmits, and a mail receiver which receives the mail from the mail transmitter. A mail receiver refers to a time limit for opening and the reply necessity added to a received mail from the mail transmitter. A receiver detects a non-opened mail which a time limit for opening has passed or a mail which a time limit for opening has passed although requiring reply, and notifies a sender (a mail transmitter) of that effect.
By this, a mail sender can know that a recipient is leaving a mail without opening or without a reply which is necessary.